The present invention relates generally to the area of tubular members of definite or indefinite length, including the wall structure of the tubular members. In particular, the present invention relates to manufacturing multi-layer tubing or pipe made from a thermoplastic material, which tubing or pipe comprises at least one inner tube or pipe that is internally positioned or situated within and separate from an outer tube or pipe.
In industries such as the semiconductor or chip processing industry where several kinds of dangerous chemicals are being used at several processing and manufacturing stages, it is important to have tubing and/or pipes to transport these chemicals and that is non-reactive with the chemicals, flexible enough to bend between connections, and provide containment in the event that the immediate tube transporting the chemical ruptures or otherwise breaks. Various materials to make these tubes and pipes are known to those skilled in the art, such as thermoplastic tubing. In this Application, unless the context indicates otherwise, the terms xe2x80x9ctubingxe2x80x9d and xe2x80x9ctubexe2x80x9d likewise refer to xe2x80x9cpipingxe2x80x9d and xe2x80x9cpipexe2x80x9d.
Common practice to produce thermoplastic tubing uses a plasticating extruder with an annular die to form a hollow tube that is pulled into a sizing device for sizing and shaping and where the thermoplastic solidifies into finished tubing. Nested tubing can be made or manufactured by various mechanical methods. Tubing having a smaller outer diameter than the inner diameter of other tubing can be manually pushed or pulled inside the other tubing. The process may be assisted by a machine. This process is generally limited to finished product lengths, however, because frictional forces cause the tubes to bind as one tube traverses the other. Lubricants can be used in some circumstances to reduce the effects of these frictional forces, but frictional forces eventually come into play again and, in many circumstance, it is undesirable to introduce the lubricant into the tubing.
Alternatively some tubing extrusion processes can produce nested tubing by producing one tube over the top of another tube. These extrusion processes rely on parts designs and/or bonding one tube to another to make multi-walled hollow tubes having rigidly interconnected walls or to hold one tube concentrically in the other tube. Examples of multi-walled hollow tubes having rigidly interconnected walls are disclosed in Hosono et al., U.S. Pat. No. 4,906,496, Double-Walled Tube Assembly and Hegler et al., U.S. Pat. No. 5,976,298, Method of Producing Multilayer Thermoplastic Pipe, both of which are incorporated herein by reference. Hosono discloses xe2x80x9cfeeding the inner tube and the extruded outer tube into a sizing die device, contracting the outer tube in a softened state toward the outer peripheral surface of the inner tube in the sizing die device until the distal ends of the ribs of the outer tube are fused to the outer peripheral surface of the inner tube, and cooling and solidifying the outer tube. Hegler discloses xe2x80x9cheating to welding temperature at least the outer surfaces of the externally profiled shape . . . and extruding a tube of plastic material on to the outer surfaces of the pipe of externally profiled shape . . . and welding together the tube and outer surfaces. and cooling the multilayer pipexe2x80x9d. Rigid tubing suffers from an inability to easily bend the tubing without creating stress cracks in the tubing.
A bonding process is exemplified by Takahashi, U.S. Pat. No. 4,157,194, Thermoplastic Multi-Walled Pipes and Takahashi, U.S. Pat. No. 4,236,953, Process for Producing Thermoplastic Multi-Walled Pipes, both of which are also incorporated herein by reference. To avoid the disadvantages of rigidly fixed multi-walled pipe or tubing, Takahashi uses xe2x80x9cribs for spacing the inner and outer tubular membersxe2x80x9d. The xe2x80x9crib . . . is so formed that the bond per unit area between the rib . . . and inner tubular member is weaker than the bond per unit area between the rib . . . and outer tubular memberxe2x80x9d so that xe2x80x9c[i]f the weak-bond temperature is properly selected at a temperature between the forming temperature and the melting temperature of resin depending upon the kinds of resin used, the inner and outer tubular members may be easily separated from each other, and the structural strength of the multi-walled pipe will not be impairedxe2x80x9d. Although these tubings are more flexible, especially when xe2x80x9cbreakawayxe2x80x9d bonds are used, there is still some stress caused by the bonding and the ribs or spaces can interfere with the bending and twisting and the operation of the tube.
Another alternative is disclosed in Wolf et al., U.S. Pat. No. 5,433,252, Fluid Containing Coaxial Tube for Control Systems, also incorporated herein by reference. Wolf discloses a coaxial tube comprising a xe2x80x9cfluid-tight outer tube and a fluid-tight inner tube with spacers between the tubes . . . [to] maintain the inner tube at least substantially coaxial with the outer tubexe2x80x9d wherein xe2x80x9cthe spacer is integral only with the outer tube or the inner tube [so] that the inner tube and the outer tube are able in a limited way to freely axially shift relative to each otherxe2x80x9d. To manufacture the coaxial tube, xe2x80x9cfirst the inner tube is manufactured separately, preferably by extrusion, and then it is cooledxe2x80x9d. The outer tube is xe2x80x9cshaped in a second extrusion station in the direction of extrusion around the inner tube, and in particular this will be done using the technique of coaxial sheath extrusionxe2x80x9d. The inner tube xe2x80x9cwill be already sufficiently cold and solid. so that, during the ensuing molding of the outer tube around the inner tube, neither the wall thickness of the inside tube will be lessened or changed, nor is the outer tube able to bond to the inner tubexe2x80x9d. The tubing in Wolf can be very flexible; however, the problem that the ribs and spacers can interfere with the bending and twisting and the operation of the tubing still exists.
It would be advantageous if an apparatus and method for manufacturing multiply contained tubing could be developed that produced both tubing that was very flexible and was ribless, i.e., the inner surface of the outer tube and the outer surface of the inner tube could be produced without ribs, spacers, or like structural features.
The present invention provides a manufacturing apparatus and method for multiple containment tubing. The apparatus produces continuous or virtually endless, nested tubing, or one tube within another tube, without having to fix the space between the tubes or position them concentrically. Consequently the tubing is very flexible and with uniform wall thickness. Moreover, because the inner surface of the outer tube and the outer surface of the inner tube are ribless, these kinds of features cannot interfere with the bending and twisting and the operation of the tubes comprising the tubing. The space between the tubes, or free space, has sufficient volume to contain a range of contents of the inner tube, as a safety containment device, if the inner tube ruptures, cracks, or otherwise breaks. In particular, resins of PFA, FEP, Olefin such as HDPE or PP materials, or other fluorinated hydrocarbons or similar materials that have chemical resistance suitable for the electronics industry or combinations of these resins are applicable to this invention.
In preferred embodiments, the apparatus comprises an outer tube extruder, an outer tube sizing device, and a hollow annular guide. The outer tube extruder includes an extruder body, a mandrel, and a die body. The mandrel and the die body have a longitudinal direction and are approximately longitudinally aligned, and the die body circumscribes the mandrel. The mandrel and the die body are connected to the extruder body so that a melt pool can be forced through the extruder body, into a cavity between the die body and the mandrel, and out from between the die body and the mandrel in the form of and to create an outer tube. A bore passes through the mandrel longitudinally and possibly through the extruder body to provide a passage for the at least one inner tube through the outer tube extruder. The outer tube sizing device also has a bore. This sizing device bore receives the outer tube form to size and shape the outer tube. An important feature of the apparatus is the annular guide. The annular guide extends at least from the outer tube extruder bore into the sizing device bore so that the at least one inner tube passes through the annular guide and the outer tube passes over or around or surrounding the annular guide. The annular guide has a wall having a distal end and a duct opening on the distal end. A duct at least passes through the wall of the annular guide to the duct opening, through which a cooling agent can pass to be applied to the inner surface of the outer tube. By cooling the inner surface of the outer tube as the outer tube leaves the annular guide, the outer tube loses its tackiness and is unlikely to bond with the at least one inner tube.
Once a preferred embodiment of the apparatus is operational, the at least one inner tube is passed through the outer tube extruder bore and annular guide, and then through the sizing device bore. A melt pool is forced through the extruder body into the cavity between the die body and mandrel and from between the die body and mandrel in the form of and to create an outer tube. The outer tube is pulled through the sizing device bore to shape it and size it. The outer tube is also cooled during shaping and sizing, using conventional means on the exterior surface of the outer tube and cooling agents applied from the annular guide opening to the interior surface of the outer tube, so that the outer tube maintains its form and to prevent it from bonding to the at least one inner tube.